This invention relates to a process for spinning and drawing continuous polyester filaments, and is more particularly concerned with production of mixed-shrinkage yarn which develops bulk due to differential shrinkage of the filaments when heated.
Yarns composed of filaments which have different heat-shrinkage characteristics are well known. When these composite yarns are heated to cause shrinkage, the filaments that shrink the most cause the lesser-shrinking filaments to take a non-linear configuration that imparts bulk to the yarn. Generally speaking, the greater the differential filament shrinkage, the greater will be the bulk of the yarn. One method of producing such yarns has been to combine two filament bundles which differ in that one bundle has been preshrunk to have less residual shrinkage than the other bundle. Conventional equipment for preparing uniform-shrinkage textile yarn can be operated to prepare two filament bundles which have different boil-off shrinkages. The different bundles can then be combined in an entirely separate operation, but the separate operation is an additional expense and it is difficult to intermingle the different filaments sufficiently to provide uniform bulk in fabrics made from the yarn.
Maerov et al. U.S. Pat. No. 3,199,281 discloses cospinning two bundles of polyester filaments, separately drawing the bundles, heat-relaxing one bundle to have a lower residual shrinkage than the other, and then combining the bundles with an interlacing jet to form a composite yarn in one continuous process. Since the two bundles must be traveling at the same speed when they are combined, compensation for shrinkage of filaments in the heat-relaxing step is provided by spinning the bundles at different speeds and/or drawing the bundles at different draw ratios. The patent also suggests the use of a "superstretching" at 130.degree. to 180.degree. C. to increase the filament length without imparting the orientation characteristic of conventionally drawn filaments. Schmitt U.S. Pat. No. 3,423,809 discloses a process in which two filament bundles are spun and drawn under identical conditions, one bundle is then annealed during passage through a steam jet device under sufficient tension to prevent shrinkage, and the bundles are then combined with an interlacing jet to form a composite yarn. The annealing treatment can provide an effective shrinkage differential between the filament bundles without having to adjust for the shrinkage which occurs in the heat-relaxing treatment mentioned previously. However, this process requires equipment for the annealing step which is not used in conventional production of continuous filament yarn.